/**
  * Returns an array containing, for each field, the arary key used for that
  * field in self::$_fields.
  *
  * @return array
  */
 static function &fieldKeys()
 {
     if (!self::$_fieldKeys) {
         self::$_fieldKeys = array('id' => 'id', 'entity_table' => 'entity_table', 'entity_id' => 'entity_id', 'financial_trxn_id' => 'financial_trxn_id', 'amount' => 'amount');
     }
     return self::$_fieldKeys;
 }